A sensitive atomic-absorption spectrometric method for the determination of tin with atomization from impregnated graphite surfaces

The atomization of Sn from graphite surfaces is potentially hindered by reactions with the surface. The impregnation of graphite tubes with other carbide-forming elements (W, Zr, Ta, Mo) favourably alters the surface characteristics of the graphite furnace for the atomization of Sn. At the acid concentrations needed to prevent the hydrolysis of Sn, these surfaces are considerably more stable (even after more than 100 atomization cycles) than those of pyrolytic graphite. Two graphite furnaces of different design, the HGA 72 and the HGA 76, were tested. With impregnated graphite tubes the determination of Sn is possible in the HGA 72 with a detection limit of approximately 15 pg. In the HGA 76 the tin determination is vastly improved with respect to prolonged lifetime of the furnaces and stable signals over much longer periods of time. Detailed interference studies reveal that the use of the "gas stop" mode minimizes the influence of many ions that are frequently either introduced by the decomposition reagents or present in the sample itself. The practical potential of this method is demonstrated for the determination of Sn in a slag material and in copper- and aluminium-based alloys.     

Mechanism of yttrium atom formation in electrothermal atomization from metallic and metal-carbide surfaces of a heated graphite atomizer in atomic absorption spectrometry

Mechanism of Y atom formation from pyrocoated graphite, tantalum and tungsten metal surfaces of a graphite tube atomizer has been studied and a mechanism for the formation for Y atoms is proposed for the first time. The following is the mechanism of yttrium atom formation regardless of whether the atomizing surface is pyrolytic graphite, tantalum or tungsten metal.

     

Effect of atomizer surface type on the quantitation of molybdenum and ytterbium by electrothermal atomization atomic absorption spectrometry

The effect of pyrocoated graphite, uncoated graphite, metal-carbide, and metal atomization surfaces on the quantitation of molybdenum and ytterbium by electrothermal atomization atomic absorption spectrometry was investigated. The peak shape was affected by heating rate and the different surfaces gave different shapes. Except for the case of uncoated graphite, the sensitivities and detection limits were similar for all surfaces. In a sodium chloride matrix it is preferable to use uncoated graphite for molybdenum because an ashing stage greater than the boiling point of sodium chloride can be used without loss of molybdenum. Tube lifetime depended on atomization temperature, atomization time and the matrix.     

Chemical modification of graphite surfaces for the determination of chromium by electrothermal atomic absorption spectrometry

Different kinds of graphite surfaces (electrographite, pyrolytic graphite, zirconium and tungsten carbide-coated) have been tested for optimization of analytical conditions for the determination of chromium using electrothermal atomic absorption spectrometry. The effect of mineral acids on the peak absorbance signal of chromium has been investigated. Considering pyrolysis temperature and sensitivity, atomization from pyrolytic graphite coated surface showed the best performance.     

Automated determination of tin by hydride generation using in situ trapping on stable coatings in graphite furnace atomic absorption spectrometry

Conditions have been studied for the determination of Sn by coupling of hydride generation and graphite furnace atomic absorption spectrometry. Sequestering and in situ concentration of Sn hydride in the graphite furnace requires just a single application of a long-term stable trapping reagent for automated analyses. In a systematic study it is shown that effective trapping of stannane is possible on graphite tubes or platforms coated with a carbide-forming element such as Zr, Nb, Ta, or W at trapping temperatures of 500 to 600°C. Trapping temperatures should not be higher than 600°C (the “critical temperature”) because otherwise at temperatures higher than 700°C errors in absorbance values could occur by an adsorptive “carry-over effect”. Signal stability and reproducibility are tested over more than 400 complete trapping and atomization cycles, and a precision of 2% is observed. Narrow peaks are obtained for all coatings except for Nb- and Ta-coated platforms where double peaks occur. Ir- or Pd/Ir-coated surfaces allow trapping of stannane at lower temperatures but the signal stability (especially in the case of Pd/Ir coating) is lower than with the carbide-forming element coatings. The highest sensitivity is found for Zr- and W-coated tubes with a characteristic mass of about 17 and 20 pg, respectively, and the calibration curves are linear up to 2 ng Sn on Zr-treated tubes (peak height) and 4 ng on Zr-coated platforms (integrated absorbance) using the 286.3 nm line. The detection limit is 25 pg for a 1 ml sample volume, and the reagent blank is still significant with the purest available chemicals. The method is tested by determination of Sn in low alloy steel samples.     

Electrothermal atomization of arsenic,antimony and thallium using a graphite atomizer with refractory metal platforms

The electrothermal atomization of the volatile elements arsenic, antimony and thallium from a refractory metal platform consisting of a tungsten coil and/or a refractory metal foil with the dimensions of a conventional graphite platform was studied. Several combinations of refractory metal platforms were investigated, as follows: W platform; Ta platform; W coil; W coil on a W platform and W coil on a Ta platform. The best combination for these elements as regards both thermal stabilization and sensitivity is the W coil on a Ta platform. Thermal stabilization is also achieved with a W coil on a W platform. The presence of Pd-containing chemical modifier favors the thermal stabilization of the analytes. The sufficient amount is 2 micrograms of Pd. The maximal temperatures of pyrolysis are higher (arsenic, antimony) or equal (thallium) to those when using different chemical modifiers, added as solutions. It may be concluded, that the refractory metal platforms act as "built-in modifiers". They are suitable for the determination of arsenic, antimony and thallium in samples of complex matrix composition where high thermal stability of the analytes during the pyrolysis step is required.     

Atomization from a tantalum surface in graphite furnace atomic absorption spectrometry

The mechanism of atom formation of U, V, Mo, Ni, Mn, Cu and Mg atomized from pyrolytic graphite and tantalum metal surfaces has been studied. The mechanism of atom formation for U from a graphite tube atomizer is reported for the first time. The peak absorbance for U and Cu is increased by factors of 59.7 and 2.0, respectively, whereas that of V, Mo and Ni is reduced by several orders of magnitude when they are atomized from a tantalum metal surface. The peak absorbance of Mn and Mg is not appreciably affected by the material of the atomization surface. Interaction of Mn and Mg with the graphite surface and formation of their refractory carbides was found to be negligible. Uranium forms a refractory carbide when heated from a graphite surface.     

Mechanism of cobalt atomization from different atomizer surfaces in graphite-furnace atomic-absorption spectrometry

The mechanism of cobalt atomization from different atomizer surfaces in graphite-furnace atomic-absorption spectrometry has been investigated. The atomizer surfaces were pyrolytically coated graphite, uncoated electrographite, and glassy carbon. The activation energy of the rate-determining step in the atomization of cobalt (taken as the nitrate in aqueous solution) in a commercial graphite furnace has been determined from a plot of log k_s vs. 1/T (for T values greater than the appearance temperature), where k_s is a first-order rate constant for atom release, and T is the absolute temperature. The activation energy E_a, can be correlated either with the dissociation energy of CoO_(g) or with the heat of sublimation of Co_(s), formed by carbon reduction of CoO_(s), the latter being the product of the thermal decomposition of Co(NO₃)₂. The mechanism for Co atomization seems to be the same for the pyrolytically coated graphite and the uncoated electrographite surfaces, but different for the glassy carbon surface. The suggested mechanisms are consistent with the chemical reactivity of the three atomizer surfaces, and the physical and thermodynamic properties of cobalt and its chemical compounds in the temperature range involved in the charring and atomization cycle of the graphite furnace.     

The graphite furnace and its role in atomic spectroscopy

The diversity of applications of the graphite furnace is extraordinary, encompassing the fields of physics, thermochemistry, spectroscopy and analytical chemistry. In this respect, the graphite furnace has been used on a continuous basis as a research tool for nearly a century. Following its introduction as an atomization source for atomic absorption spectrometry by L'vov in 1959, its role in atomic spectrometry expanded considerably to encompass analytical applications in emision, fluorescence, absorption and mass spectrometry. In addition to its conspicuous use as an atomization source in these areas, it is frequently employed as a vaporizer when used in the format of combined and tandem sources with other instrumentation. The unique physico-chemical micro-environment which can be attained within the graphite furnace has also been used to advantage in a number of investigations, including the determination of gas- and solid-phase diffusion coefficients of high-temperature metal vapours, the heats of sublimation of refractory metals, fundamental optical constants and the measurement of the heats of desorption of adatoms from high-temperature surfaces. The range of such applications remains to be more fully explored. The attractive features of this source, viz., the high atomization/vaporization efficiency, comparatively long atomic vapour residence times, controllable chemical and thermal environment and its ability to handle high dissolved solids content samples (相似文献   

Effect of magnesium nitrate vaporization on gas temperature in the graphite furnace

The vaporization of magnesium nitrate was observed in longitudinally-heated graphite atomizers, using pyrocoated and Ta-lined tubes and filter furnace, Ar or He as purge gas and 10–200-μg samples. A charge coupled device (CCD) spectrometer and atomic absorption spectrometer were employed to follow the evolution of absorption spectra (200–400 nm), light scattering and emission. Molecular bands of NO and NO₂ were observed below 1000°C. Magnesium atomic absorption at 285.2 nm appeared at approximately 1500°C in all types of furnaces. The intensity and shape of Mg atomization peak indicated a faster vapor release in pyrocoated than in Ta-lined tubes. Light scattering occurred only in the pyrocoated tube with Ar purge gas. At 1500–1800°C it was observed together with Mg absorption using either gas-flow or gas-stop mode. At 2200–2400°C the scattering was persistent with gas-stop mode. Light scattering at low temperature showed maximum intensity near the center of the tube axis. Magnesium emission at 382.9, 383.2 and 383.8 nm was observed simultaneously with Mg absorption only in the pyrocoated tube, using Ar or He purge gas. The emission lines were identified as Mg ³P°–³D triplet having 3.24 eV excitation energy. The emitting species were distributed close to the furnace wall. The emitting layer was thinner in He than in Ar. The experimental data show that a radial thermal gradient occurs in the cross section of the pyrocoated tube contemporaneously to the vaporization of MgO. This behavior is attributed to the reaction of the sample vapor with the graphite on the tube wall. The estimated variation of temperature within the cross section of the tube reaches more than 300–400°C for 10 μg of magnesium nitrate sampled. The increase of gas temperature above the sample originates a corresponding increase of the vaporization rate. Fast vaporization and thermal gradient together cause the spatial condensation of sample vapor that induces the light scattering.     

Alkali metal fluoride matrix modifiers for the determination of trace levels of silicon by graphite furnace atomic absorption spectrometry

A method is described for the determination of silicon by graphite-furnace atomic absorption spectrometry with alkali metal fluorides as matrix modifiers. Alkali metal fluorides react with silicon to produce metal hexafluorosilicates, which are stable at temperatures as high as 1120– 1300°C, depending on the particular metal used. At higher temperatures, alkali metal hexafluorosilicates decompose into the metal fluoride and silicon tetrafluoride. The subsequent gas-phase atomization of silicon tetrafluoride occurs rapidly, and produces clean, sharp absorption peaks. When used in conjunction with a zirconium-treated graphite tube, this method has a sensitivity of 50 pg Si at a confidence interval of 95%. There is no evidence of silicon carbide formation in the graphite tube, and very little evidence of any deterioration of the zirconium-treated surface, as demonstrated by the fact that more than 200 samples can be processed on a single graphite tube without a decrease in sensitivity or change in the baseline.     

Determination of thallium by furnace atomic absorption spectrometry

The analytical conditions for the determination of thallium by graphite furnace atomic absorption spectrometry were studied and optimized using the peak-height mode. The charring-atomization curves for thallium from different atomization surfaces were constructed and the optimum charring and atomization conditions were established. These atomization surfaces included pyrolytic graphite-, tantalum-, zirconium- and tungsten-coated graphite tubes. The effects of different inorganic acids on the absorbance of thallium from different surfaces were studied. Using tungsten carbide-coated tubes, the interference effects due to hydrochloric and perchloric acids were eliminated. The matrix modification technique was also investigated for increasing the maximum permissible charring temperature for thallium. The matrix modifiers used included tungsten, zirconium, nickel and tantalum. The effect of adding these modifiers were studied in the presence of different acids. Tungsten increased the maximum permissible charring temperature from 400 to 1000 °C.     

Aspects of the pyrometric measurement of graphite furnace temperature

Experimental results are presented to show that the radiation leaving the centre hole of tubular graphite furnace atomisers can deviate significantly from ideal blackbody behaviour. Pyrometric measurements at various wavelengths of the apparent, or brightness, temperatures of two commercial graphite tube atomisers (Varian GTA-95 and CRA-90) showed that the measured brightness temperature decreased appreciably with increasing wavelength, in accordance with the predicted behaviour for a cavity radiator with an emissivity less than unity. Measurement of the sample hole emissivity from the apparent melting temperatures of certain metal and metal carbide samples indicated mean effective hole emissivities of pyrocoated GTA-95 tubes in the range 0.76–0.92, depending on temperature and wavelength. Implications of the results for pyrometric temperature measurement in graphite furnaces are also discussed.     

不同表面石墨管测定比较及采用WPGT直接测定水系沉积物及煤飞灰中的钇

本文采用自制的钨金属表面石墨管(WPGT)测钇,并与其它石墨管进行了比较。WPGT测钇灵敏度高,为PGT的340倍,重现性好,抗干扰能力强,可用于水系沉积物等样品中钇的直接测定。     

Effect of beryllium nitrate vaporization on surface temperature in the pyrocoated graphite furnace

The vaporization of 20–50 μg beryllium from nitrate solution was observed in graphite furnace atomizers using pyrocoated and Ta-lined tubes. A charge coupled device (CCD) spectrometer was employed to follow the evolution of absorption spectra (200–475 nm), the light scattering and emission. Molecular bands of NO and NO₂ were observed below 1000°C. Beryllium absorption at 234.9 nm was prominent in spectra above 2200°C and 1900°C, respectively, in Ta-lined and pyrocoated tubes. The evolution profile of Be atomic absorption and of some bands indicated a faster vapor release in the pyrocoated tube. Light scattering occurred only in the pyrocoated tube, increasing with the tube age. When purge gas mini flow was applied, the scattering was observed at 1900–2200°C simultaneously with Be atomic absorption and emission continuum at long wavelength. The emission continuum showed the wavelength distribution characteristic of black body radiation. The temperature increase, due to the vaporization of the sample, was estimated using Planck’s equation. The maximum temperature increase reached 400°C, when the most intense Be atomic absorption, light scattering and emission was observed. According to the hypothesis proposed, the black body radiation was induced by the formation of Be carbide in the pyrographite layer. Low heat capacity across the pyrographite prevented the heat dissipation, and led to increase of surface temperature. This induced an increase of sample evaporation rate and the formation of a thermal gradient in the cross section of the tube. Both factors originated vapor supersaturation in the tube center, spatial condensation and, accordingly, light scattering. The results, together with those already obtained with Mg nitrate place limitations to the atomization theories based on the concept of isothermal equilibrium or on Arrhenius kinetic approach.     

Factors Affecting Selenium Atomization Efficiency in Graphite Furnace Atomic Absorption

Abstract

The effect of graphite furnace surface treatment, and the addition of “matrix modifiers” such as nickel or lanthanum on the observed selenium atomization siqnal in electrotherval atomic absorption analysis has been investiqated. The results indicate that the removal of signal depression caused by the addition of metal solution to analyte solution is not simply a devolatilization of the selenium. The effect appears to be a modification of the graphite surface which leads to more efficient atom formation. The role of the surface was investigated monitoring the atomic absorption signal generated from graphite furnaces which were untreated, pyrolytic graphite-coated, zirconium-or tantalum-coated, and metal-coated followed by pyrolytic graphite coating. The dependence of the analyte signal on the concentration of added metal was investigated for these surfaces. The optimum results obtained were the metal-coated/pyrolytic graphite-coated cuvettes. These cuvettes showed reduced effect of “matrix modifier,” suggesting that the surface treatment can replace the “matrix modifer,” Surface chemistry consistent with the atomic absorption observations and surface analysis data is presented.     

Determination of lanthanum in food and water samples by Zeeman-effect atomic absorption spectrometry using graphite tube lined with tungsten foil.

A sensitive, selective method for the determination of lanthanum in food and water samples by atomic absorption spectrometry using a graphite tube lined with tungsten foil is described. The atomization of lanthanum from the tungsten surface gives better analytical sensitivity, a lower atomization temperature and negligible memory effects. The characteristic mass and detection limit of the method were 8.1 x 10(-9) and 7.85 x 10(-9) g, respectively. The precision (relative standard deviation in the range 5.9-9.9%), accuracy and interferences of the method were also investigated. The method can be used directly for the determination of trace amounts of lanthanum in food and water without pre-dissociation of the matrices. The results obtained by this method are in good agreement with those obtained from inductively coupled plasma atomic emission spectrometry.     

Simulation of atomic absorption signals: A kinetic model with two independent sources

A kinetic model of atomization processes based on the solution of one-dimensional diffusion equation with two independent sources is proposed. One of the sources describes the atomization of atoms from the graphite furnace surface, while another one describes the atom formation inside the walls of the furnace and their subsequent outflow into the analytical zone. This mechanism is used to describe electrothermal atomization of Cu and Ag. The simulations show that the form of atomic absorption signal of Cu is determined to the great extent by the processes of desorption from the graphite surface and diffusion inside the graphite. The tailing of the back edge of absorption profile can be explained by the rather slow diffusion process of copper atoms in the graphite. At the same time, for the atomization of Ag, the process of separation of single atoms from clusters is the limiting process.A new interpretation for the shift of absorbance maximum as the initial mass of Ag increases is proposed.     

元素在石墨炉内石墨探针表面上的原子化机理研究(X)——硝酸锶的原子化机理

用X射线衍射、X射线光电子能谱、俄歇电子能谱和扫描电子显微术等考察了石墨炉升温过程中Sr(NO₃)₂在石墨探针表面上的形态变化,阐明了它的原子化机理.加热过程中Sr(NO₃)₂首先分解为SrO(s),再还原为SrC₂,后者进一步分解为Sr(s).锶的原子化源于金属蒸发.